<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<script type="text/javascript">
			function $$(id){
				return document.getElementById(id);
			}
			window.onload=function(){
				var cnv=$$("canvas");
				var cxt=cnv.getContext("2d");
				createPolygon(50,50,5,40,cxt);
				var movex,movey;
				
				//绘制一个正多边形函数
				function createPolygon(dx,dy,n,size,cxt){
					cxt.beginPath();
					for (var i=0;i<n;i++){
						var x=size*Math.cos(i*2*Math.PI/n)+dx;
						var y=size*Math.sin(i*2*Math.PI/n)+dy;
						cxt.lineTo(x,y);
					}
					cxt.fillStyle="red";
					cxt.fill();
					cxt.closePath();
				}
				canvas.addEventListener("mousedown",downfun);
				
				function downfun(e){
					var x=e.clientX;
					var y=e.clientY;
					if (cxt.isPointInPath(x,y)){
						canvas.addEventListener("mousemove",movefun);
					}
					canvas.addEventListener("mouseup",upfun);
				}
				function movefun(e){
					movex=e.clientX;
					movey=e.clientY;
					cxt.clearRect(0,0,canvas.width,canvas.height);
					createPolygon(movex,movey,8,40,cxt);
					
				}
				function upfun(){
					canvas.removeEventListener("mousemove",movefun);
				}
			}
			
			
			
		</script>
	</head>
	<body>
		<canvas id="canvas" width="600" height="400" style="border: 1px dashed gray;"></canvas>
	</body>
</html>
